The present disclosure relates to a hydraulic control circuit for a construction machine with a hydraulic cylinder that raises and lowers heavy loads.
There exists a construction machine such as a hydraulic shovel provided with various hydraulic actuators such as a hydraulic cylinder that raises and lowers heavy loads; a control valve that controls an oil supply and discharge to/from the hydraulic actuators based on an operation of operating units; and a hydraulic pump as a hydraulic supply source. When a hydraulic actuator is a boom cylinder for raising and lowering a boom of a hydraulic shovel, for example, the boom cylinder extends to raise the boom through an oil supply to a head-side oil chamber as a weight holding-side oil chamber and an oil discharge from a rod-side oil chamber as an anti-weight holding-side oil chamber. The boom cylinder also retracts to lower the boom through an oil supply to the rod-side oil chamber and an oil discharge from the head-side oil chamber.
In order to lower the boom, a weight that is applied to the boom (a total weight of a front working part) acts as a force by which the boom cylinder retracts, with a resultant pressure in the head-side oil chamber higher than the rod-side oil chamber. Accordingly, there is provided a recovery oil passage in which discharged oil from the head-side oil chamber is supplied as recovery oil to the rod-side oil chamber when the boom is lowered. Such recovery oil and pressure oil that is supplied from the hydraulic pump are configured to be supplied to the rod-side oil chamber while pressure is higher in the head-side oil chamber than the rod-side oil chamber.
In the arrangement in which the recovery oil passage is provided, a control valve is neutralized when the boom is lowered (see Japanese Published Unexamined Patent Application No. A-09-132927, for example), and a switching controls a supply flow rate from the hydraulic pump to the rod-side oil chamber when the boom is lowered (see Japanese Published Unexamined Patent Application No. A-2005-256895, for example). Recovery oil is supplied to the rod-side oil chamber from the head-side oil chamber while no pressure oil is supplied from the hydraulic pump in order to lower the boom. Accordingly, a discharge flow rate of the hydraulic pump can be reduced.